1. Field of the Invention
The present invention relates to methods for making fiber-reinforced composite articles where uncured resin is infused into a fibrous body located in a mold to form an uncured resin impregnated fibrous body that is then cured. More particularly, the present invention involves controlling the flow of resin during the molding process to prevent or at least reduce unwanted flow or migration of the resin within the fibrous body and mold.
2. Description of Related Art
Fiber-reinforced composites are widely used to form structures and components for use in a range of different industries including, for example, the aerospace, transport, electronics, building and leisure industries.
A common approach for the preparation of such structures or components, especially large structures or components such as boat hulls, wind turbine blades, or certain aircraft components, involves the arrangement of a fibrous reinforcement assembly within a mold followed by the impregnation or infusion of the assembly with a mixture of one or more liquid resins with one or more curing agents. Once the impregnation or infusion of the liquid resin/curing agent mixture into the assembly is complete, the liquid resin is then cured to yield the final molded structure or component. Curing is typically accomplished by heating the assembly.
There are a number of different techniques by which a resin/curing agent mixture may be infused or impregnated into the fibrous reinforcement assembly. Conventional techniques involve either brushing or metering the resin/curing agent mixture onto the fibrous reinforcement. Although such techniques are simple and relatively inexpensive to implement in practice, the resultant product can be variable in quality and the mechanical properties of the final structure or component can often be poor.
More recently, liquid resin infusion technologies, such as resin transfer molding (RTM), vacuum-assisted resin transfer molding (VaRTM) and resin infusion using flexible tooling (RIFT), have been developed. All of these technologies rely on the basic concept of injecting or infusing the resin into the fibrous reinforcement assembly, either in a closed mold in the case of an RTM process, or in a vacuum bag molding in the case of a VaRTM process. These processes carry numerous advantageous over the conventional techniques by virtue of the improved hygiene and safety with which these mainly enclosed processes can be carried out, the favorable mechanical properties of the final composite component or structure, and favorable manufacture costs.
When preparing fiber-reinforced components by a liquid resin infusion process, it has been observed that there is a tendency for the liquid resin infused into the fibrous reinforcement assembly to become depleted within certain areas of the assembly during infusion and cure. This occurrence is especially common when a vacuum resin infusion process is used to prepare large components, such as a wind turbine blade or boat hull, for example. Resin depletion is also a problem when the liquid resin infused into the fibrous reinforcement assembly is of particularly low viscosity. Low viscosity resins provide a number of particular benefits including relatively high infusion speed, for example. The occurrence of resin depletion can be particularly disadvantageous because the final structure or component formed following the cure of the liquid resin does not contain the originally intended distribution of resin throughout its structure. As a consequence, the mechanical properties and performance of the structure or component thus formed can be compromised, particularly in the regions where resin depletion has occurred. The occurrence of resin depletion can often be visualized by the presence of white patches on the surface of glass-reinforced components. These may appear during infusion or may only become apparent following curing.
In the case of a VaRTM process, for example, the areas of the assembly that have been observed to be particularly prone to the occurrence of resin depletion include the areas in the vicinity of the suction tubing that connects the vacuum pump to the mold. In these areas, resin depletion tends to occur as a result of the suction force generated by the vacuum pump. Resin depletion can also result from the effect of gravity in areas that are in contact with vertically inclined sections of the mold. Resin depletion in these areas is especially likely to occur once all the interstices between fibers of the fibrous reinforcement have become fully saturated with the infused liquid resin. Furthermore, the support pressure applied to the assembly by the vacuum bag is typically less than 1 bar (100 kPa), which in itself is insufficient to prevent the liquid resin flowing out of the vertically inclined sections of the composition under the force of gravity.
If the resin is an epoxy resin, the problem is further exacerbated when the assembly is heated to cause the resin to cure because the viscosity of the epoxy resins can decrease significantly during the cure cycle. Although common to other resin systems, liquid epoxy resins are less likely to be able to counteract this viscosity decrease by a rapid gelation and cure mechanism.
This temporarily unchecked reduction in viscosity, particularly when coupled with the fact that it is often necessary to maintain the vacuum suction to the assembly once the resin infusion is complete and also during the cure cycle, further increases the tendency for the resin depletion to occur, particularly in susceptible areas.
One approach to solve this problem is to use a very rapid curing system; the gel time for formulated epoxy resin infusion systems can be reduced to just a few minutes or, in some cases, even less. However, this can impact severely on the viscosity of the composition during the infusion process, and the safety of the operation, such high reactivity often being accompanied by high exothermic heat generation.
It is an object of the present invention, therefore, to provide a process and composition for the preparation of a fiber-reinforced composite structure or component by a resin infusion process in which depletion of resin in areas that are prone to resin depletion is significantly decreased or eliminated altogether.